Large amounts of folic acid can hide a vitamin B12 deficiency

The following info is from:

http://ods.od.nih.gov/factsheets/VitaminB12-QuickFacts/

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Large amounts of folic acid can hide a vitamin B12 deficiency by correcting megaloblastic anemia, a hallmark of vitamin B12 deficiency. But folic acid does not correct the progressive damage to the nervous system that vitamin B12 deficiency also causes. For this reason, healthy adults should not get more than 1,000 mcg of folic acid a day.